The data on sex, which was asked of all people, were derived from answers to long-form questionnaire Item 3 and short-form questionnaire Item 5. Individuals were asked to mark either "male" or "female" to indicate their sex. For most cases in which sex was not reported, it was determined from the persons given (i.e., first) name and household relationship. Otherwise, sex was imputed according to the relationship to the householder and the age of the person. (For more information on imputation, see "Accuracy of the Data.")

The data on age, which was asked of all people, were derived from answers to the long-form questionnaire Item 4 and short-form questionnaire Item 6. The age classification is based on the age of the person in complete years as of April 1, 2000. The age of the person usually was derived from their date of birth information. Their reported age was used only when date of birth information was unavailable.

Data on age are used to determine the applicability of some of the sample questions for a person and to classify other characteristics in census tabulations. Age data are needed to interpret most social and economic characteristics used to plan and examine many programs and policies. Therefore, age is tabulated by single years of age and by many different groupings, such as 5-year age groups.

Median age divides the age distribution into two equal parts: one-half of the cases falling below the median age and one-half above the median. Median age is computed on the basis of a single year of age standard distribution (see the "Standard Distributions" section under "Derived Measures"). Median age is rounded to the nearest tenth. (For more information on medians, see "Derived Measures".)

The most general limitation for many decades has been the tendency of people to overreport ages or years of birth that end in zero or 5. This phenomenon is called "age heaping." In addition, the counts in the 1970 and 1980 censuses for people 100 years old and over were substantially overstated. So also were the counts of people 69 years old in 1970 and 79 years old in 1980. Improvements have been made since then in the questionnaire design and in the imputation procedures that have minimized these problems.

Review of detailed 1990 census information indicated that respondents tended to provide their age as of the date of completion of the questionnaire, not their age as of April 1, 1990. One reason this happened was that respondents were not specifically instructed to provide their age as of April 1, 1990. Another reason was that data collection efforts continued well past the census date. In addition, there may have been a tendency for respondents to round their age up if they were close to having a birthday. It is likely that approximately 10 percent of people in most age groups were actually 1 year younger. For most single years of age, the misstatements were largely offsetting. The problem is most pronounced at age zero because people lost to age 1 probably were not fully offset by the inclusion of babies born after April 1, 1990. Also, there may have been more rounding up to age 1 to avoid reporting age as zero years. (Age in complete months was not collected for infants under age 1.)

The reporting of age 1 year older than true age on April 1, 1990, is likely to have been greater in areas where the census data were collected later in calendar year 1990. The magnitude of this problem was much less in the 1960, 1970, and 1980 censuses where age was typically derived from respondent data on year of birth and quarter of birth.

These shortcomings were minimized in Census 2000 because age was usually calculated from exact date of birth and because respondents were specifically asked to provide their age as of April 1, 2000. (For more information on the design of the age question, see the section below that discusses "Comparability.")

Age data have been collected in every census. For the first time since 1950, the 1990 data were not available by quarter year of age. This change was made so that coded information could be obtained for both age and year of birth. In 2000, each individual has both an age and an exact date of birth. In each census since 1940, the age of a person was assigned when it was not reported. In censuses before 1940, with the exception of 1880, people of unknown age were shown as a separate category. Since 1960, assignment of unknown age has been performed by a general procedure described as "imputation." The specific procedures for imputing age have been different in each census. (For more information on imputation, see "Accuracy of the Data.")

All people classified in the civilian labor force (i.e., "employed" and "unemployed" people), plus members of the U.S. Armed Forces (people on active duty with the United States Army, Air Force, Navy, Marine Corps, or Coast Guard).

Data on veteran status, period of military service, and years of military service were derived from answers to long-form questionnaire Item 20, which was asked of a sample of the population 15 years old and over.

The data on veteran status were derived from answers to long-form questionnaire Item 20a. For census data products, a "civilian veteran" is a person 18 years old and over who, at the time of the enumeration, had served on active duty in the U.S. Army, Navy, Air Force, Marine Corps, or Coast Guard in the past (even for a short time), but was not then on active duty, or who had served in the Merchant Marine during World War II. People who had served in the National Guard or Military Reserves were classified as veterans only if they had ever been called or ordered to active duty, not counting the 4 to 6 months for initial training or yearly summer camps. All other civilians 18 years old and over were classified as nonveterans.

People who indicated in long-form questionnaire Item 20a that they had served on active duty in the past (civilian veterans) or were on active duty at the time of enumeration were asked to indicate in Question 20b the period or periods in which they served. People who served in both wartime and peacetime periods are tabulated according to their wartime service.

The responses to the question about period of service were edited for consistency and reasonableness. The edit eliminated inconsistencies between reported period(s) of service and the age of the person; it also removed reported combinations of periods containing unreasonable gaps (for example, it did not accept a response that indicated that the person had served in World War II and in the Vietnam era, but not in the Korean conflict).

People who indicated in long-form questionnaire Item 20a that they had served on active duty in the past (civilian veterans) or were on active duty at the time of enumeration were asked whether they had spent at least 2 years in total on active duty. The question asked for accumulated service (i.e., total service), which is not necessarily the same as continuous service. The years of military service question provides necessary information to estimate the number of veterans that are eligible to receive specific benefits.

There may be a tendency for the following kinds of people to report erroneously that they had served on active duty in the armed forces: (a) people who served in the National Guard or Military Reserves, but were never called to active duty; (b) civilian employees or volunteers for the USO, Red Cross, or the Department of Defense (or its predecessors, the Department of War and the Department of the Navy); and (c) employees of the Merchant Marine or Public Health Service. There is also the possibility that people may have misreported years of service in long-form questionnaire Item 20c because of rounding errors (for example, people with 1 year 8 months of active duty military service may have mistakenly reported "2 years or more").

Since census data on veterans are based on self-reported responses, they may differ from data from other sources, such as administrative records of the Department of Defense and/or the Department of Transportation. Census data also may differ from Department of Veterans Affairs data on the benefits-eligible population, since criteria for determining eligibility for veterans benefits differ from the rules for classifying veterans in the census.

The questions and concepts for veterans data for Census 2000 were essentially the same as those used for the 1990 census, with the following exceptions: (1) the period of military service categories were updated; (2) in an effort to reduce reporting error, the format of the years of military service question was changed from an open-ended one (how many years has...served?) to a closed-ended one (the respondent checked either of two boxes: less than 2 years/2 years or more); and (3) persons with service during World War II in the Womens Air Forces Service Pilots organization were first counted as veterans in Census 2000, a development that should not appreciably affect 1990-2000 comparability. Both the 2000 and 1990 veteran-status questions represented expanded versions of the corresponding question in the 1980 census, which asked only whether the person was a veteran or not. The expansion was intended to clarify the appropriate response for persons currently in the armed forces and for persons whose only military service was for training in the Reserves or National Guard.

People may have chosen to provide two or more races either by checking two or more race response check boxes, by providing multiple write-in responses, or by some combination of check boxes and write-in responses. The race response categories shown on the questionnaire are collapsed into the five minimum races identified by the OMB, and the Census Bureau "Some other race" category. For data product purposes, "Two or more races" refers to combinations of two or more of the following race categories:

White

Black or African American

American Indian and Alaska Native

Asian

Native Hawaiian and Other Pacific Islander

Some other race

There are 57 possible combinations (see below) involving the race categories shown above. Thus, according to this approach, a response of "White" and "Asian" was tallied as two or more races, while a response of "Japanese" and "Chinese" was not because "Japanese" and "Chinese" are both Asian responses. Tabulations of responses involving reporting of two or more races within the American Indian and Alaska Native, Asian, or Native Hawaiian and Other Pacific Islander categories are available in other data products.

1. White; Black or African American
2. White; American Indian and Alaska Native
3. White; Asian
4. White; Native Hawaiian and Other Pacific Islander
5. White; Some other race
6. Black or African American; American Indian and Alaska Native
7. Black or African American; Asian
8. Black or African American; Native Hawaiian and Other Pacific Islander
9. Black or African American; Some other race
10. American Indian and Alaska Native; Asian
11. American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
12. American Indian and Alaska Native; Some other race
13. Asian; Native Hawaiian and Other Pacific Islander
14. Asian; Some other race
15. Native Hawaiian and Other Pacific Islander; Some other race
16. White; Black or African American; American Indian and Alaska Native
17. White; Black or African American; Asian
18. White; Black or African American; Native Hawaiian and Other Pacific Islander
19. White; Black or African American; Some other race
20. White; American Indian and Alaska Native; Asian
21. White; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
22. White; American Indian and Alaska Native; Some other race
23. White; Asian; Native Hawaiian and Other Pacific Islander
24. White; Asian; Some other race
25. White; Native Hawaiian and Other Pacific Islander; Some other race
26. Black or African American; American Indian and Alaska Native; Asian
27. Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
28. Black or African American; American Indian and Alaska Native; Some other race
29. Black or African American; Asian; Native Hawaiian and Other Pacific Islander
30. Black or African American; Asian; Some other race
31. Black or African American; Native Hawaiian and Other Pacific Islander; Some other race
32. American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
33. American Indian and Alaska Native; Asian; Some other race
34. American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some other race
35. Asian; Native Hawaiian and Other Pacific Islander; Some other race
36. White; Black or African American; American Indian and Alaska Native; Asian
37. White; Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander
38. White; Black or African American; American Indian and Alaska Native; Some other race
39. White; Black or African American; Asian; Native Hawaiian and Other Pacific Islander
40. White; Black or African American; Asian; Some other race
41. White; Black or African American; Native Hawaiian and Other Pacific Islander; Some other race
42. White; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
43. White; American Indian and Alaska Native; Asian; Some other race
44. White; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some other race
45. White; Asian; Native Hawaiian and Other Pacific Islander; Some other race
46. Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
47. Black or African American; American Indian and Alaska Native; Asian; Some other race
48. Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some other race
49. Black or African American; Asian; Native Hawaiian and Other Pacific Islander; Some other race
50. American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some other race
51. White; Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander
52. White; Black or African American; American Indian and Alaska Native; Asian; Some other race
53. White; Black or African American; American Indian and Alaska Native; Native Hawaiian and Other Pacific Islander; Some other race
54. White; Black or African American; Asian; Native Hawaiian and Other Pacific Islander; Some other race
55. White; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some other race
56. Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some other race
57. White; Black or African American; American Indian and Alaska Native; Asian; Native Hawaiian and Other Pacific Islander; Some other race

Given the many possible ways of displaying data on two or more races, data products will provide varying levels of detail. The most common presentation shows a single line indicating "Two or more races." Some data products provide totals of all 57 possible combinations of two or more races, as well as subtotals of people reporting a specific number of races, such as people reporting two races, people reporting three races, and so on.

In other presentations on race, data are shown for the total number of people who reported one of the six categories alone or in combination with one or more other race categories. For example, the category, "Asian alone or in combination with one or more other races" includes people who reported Asian alone and people who reported Asian in combination with White, Black or African American, Native Hawaiian and Other Pacific Islander, and Some other race. This number, therefore, represents the maximum number of people who reported as Asian in the question on race. When this data presentation is used, the individual race categories will add to more than the total population because people may be included in more than one category.